An improved method for the quantitative derivatization of amino acids with fluorenylmethyl chloroformate (FMOC-Cl) is described. Amino acids are derivatized in borate buffer at pH 11.4 for 40 min at ambient temperature. All amino acids resulted in stable derivatives. In particular, improved derivatization was obtained with the troublesome amino acids His and Tyr: exclusively monosubstituted His and disubstituted Tyr were formed, eluting as free peaks in the chromatogram. These derivatives show a higher fluorescence response than their disubstituted and monosubstituted counterparts, respectively, resulting from other protocols. Under the new conditions, considerable less of the hydrolysis product of FMOC-Cl is seen in the chromatograms. Baseline noise was substantially reduced at a higher emission wavelength (630 nm instead of 313 or 340 nm). With simple precautions, extensive adsorption of the disubstituted derivatives (Lys, Hyl, and Tyr) on plastic or glass surfaces could be prevented. Calibration curves were linear over a 10 to 300 molar ratio of FMOC-Cl to total amino acid. The detection limits are in the femtomole range and the derivatives are stable for more than 48 h, thus permitting automated analysis of multiple samples.